Exoantigens are produced by pathogenic and nonpathogenic trypanosomes such as Trypanosoma brucei and T. lewisi, respectively. These antigens elicit a protective, trypanocidal response, but one of the exoantigens of T. lewisi is ablastinogen, which elicits the ablastic response, a reproduction-inhibiting immunity that distinguishes the nonpathogenic from the pathogenic African trypanosomes. It is proposed to continue the comparative study of exoantigens and other associated antigens in the cell surface of pathogenic trypanosomes in a single host species. This will include characterization of ablastinogen from T. lewisi and a search for a counterpart in T. brucei; completion of cytochemical studies of the cell surface of T. brucei to complement those done with T. lewisi; completion of studies of the T. brucei; continuation of studies of cap formation with all known surface ligands in T. lewisi and T. brucei; a study of cell coat formation, and its possible relation to antigenic variation in T. brucei, with immunofluorescence and capping techniques: and analysis of the cell surface of antigenic variants of T. brucei for shared, accessible antigens that are not ordinarily immunogenic. Through this comparative approach, it may be possible to learn why an ablatic sponse fails to develop against the pathogenic trypanosomes. Furthermore, the proposed study will provide basic information about the biology of these parasites and may reveal significant properties that could lead to effective control measures against the African trypanosomes.